1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine or a printer, using an electrophotography method or an electrostatic recording method. More particularly, the present invention relates to an image forming apparatus including a pull controlling mechanism of a recording-material supporting belt or of an intermediate transfer belt, disposed adjacent to an image bearing member that bears a toner image.
2. Description of the Related Art
Hitherto, for example, as a color image forming apparatus capable of forming a full-color image, the following image forming apparatuses of a direct transfer type or an intermediate transfer type are known. In the direct transfer type, toner images formed on a plurality of photosensitive drums are transferred onto a transfer member that is supported by a rotatable belt member (hereunder referred to as “transfer belt”) serving as a supporting member that supports the transfer member. In the intermediate transfer method, toner images formed on a plurality of photosensitive drums are subjected to a primary transfer operation, that is, are temporarily transferred onto a rotatable belt member (hereunder referred to as “intermediate transfer belt”) serving as an intermediate transfer member. Then, the toner images on the intermediate transfer belt are subjected to a secondary transfer operation, that is, are transferred onto a recording material. The intermediate transfer method facilitates forming of an image on various transfer members, and can increase selectivity of recording materials.
Control of Pull of Belt Member
When the image forming apparatus is operating, it is possible for any of these belt members to meander, and to become pulled from its predetermined position when, for example, a difference in the perimeter of the belt member, itself, or a misalignment between a plurality of belt supporting rollers occurs, due to, for example, a deformation of a main body of the apparatus.
As a method of correcting the belt pull, Japanese Patent Laid-Open No. 2000-266139 discusses a method of detecting a pull amount of a belt by detecting the position of an edge of the belt, and correcting an inclination angle of one of the supporting rollers on the basis of detection information. This method makes it possible to considerably increase belt life with less mechanical stress compared to a method that controls a rib-like rubber adhered to a belt edge or that controls the belt edge by directly abutting it against, for example a flange.
FIG. 15 schematically shows a related image forming apparatus using an intermediate transfer method. Four process units, which are image forming devices, are provided in correspondence with respective colors, yellow, magenta, cyan, and black. Reference numerals 1a to 1d denote photosensitive drums, reference numerals 2a to 2d denote changing devices, reference numerals 3a to 3c denote exposing devices, symbols 4a to 4d denote developing devices, reference numeral 51 denotes an intermediate transfer belt, reference numerals 53a to 53d denote primary transfer members, and reference numerals 6a to 6d denote photosensitive drum cleaners. Reference numeral 55 denotes a steering roller, reference numeral 56 denotes a driving roller for rotating the intermediate transfer belt, reference numerals 56 and 57 denote secondary transfer members, and reference numeral 140 denotes a belt edge detector.
In the image forming apparatus shown in FIG. 15, a pull amount of the intermediate transfer belt 51 is detected by the belt edge detector 140, and an inclination angle of the steering roller 55 is adjusted. In an inclination angle method, either one of two axes at respective ends of the steering roller is moved in the direction of the arrow shown in FIG. 15 (that is, substantially vertically).
Separation of Intermediate Transfer Belt
In a color image forming apparatus, an image may be formed using any one of the image bearing members. That is, an image may be formed using only one color, such as black. Here, if, for example, consumption of the image bearing members or other related members is considered, it is desirable that the image bearing members for the other colors not involved in the image formation be stopped. However, if the other photosensitive drums are stopped during rotation of the intermediate transfer belt, the photosensitive drums are scratched due to rubbing. In contrast, Japanese Patent Laid-Open Nos. 2004-117426, 2005-62642, 2002-173245, and 2003-337454 discuss a structure in which image bearing members other than a black image bearing member are separated from a transfer belt or an intermediate transfer belt when only a black image is to be formed.
The structure of separating the intermediate transfer belt will be described using FIG. 16. An image forming apparatus shown in FIG. 16 has a structure that is the same as that of the image forming apparatus shown in FIG. 15. FIG. 16 shows a state in which a primary transfer section is separated.
First, when a full-color image is to be formed, after uniformly charging photosensitive drums 1a to 1d by charging devices 2a to 2d, the photosensitive drums 1a to 1d are subjected to exposure by exposing devices 3a to 3d in accordance with an image signal, so that electrostatic latent images are formed on the photosensitive drums 1a to 1d. Thereafter, toner images are developed by developing devices 4a to 4d, so that the toner images on the photosensitive drums 1a to 1d are successively transferred onto an intermediate transfer belt 51 by applying a transfer bias to transfer members 53a to 53d from a transfer high-voltage source (not shown). At this time, by disposing a regulating roller 58, which regulates the position of the intermediate transfer belt, at a position A (indicated by a broken line), the intermediate transfer belt is disposed in contact with the photosensitive drums of the four colors (as indicated by a broken line). Transfer residual toner remaining on the photosensitive drums 1a to 1d is collected by photosensitive drum cleaners 6a to 6d. The images that are successively multiplexed and transferred onto the intermediate transfer belt 51 from the respective photosensitive drums in the aforementioned manner are transferred onto a recording material P by applying a secondary transfer bias between secondary transfer members 56 and 57. Fixing the toner images on the recording material P by a fixing device 7 causes the full-color image to be formed.
When a black single-color image is to be formed, for separating the intermediate transfer belt from the photosensitive drums 1a, 1b, and 1c (used to form yellow, magenta, and cyan images, respectively), the regulating roller 58, which regulates the position of the intermediate transfer belt, is disposed at a position B. This causes the intermediate transfer belt to be disposed at a position indicated by a solid line in FIG. 16. The black single-color image is only formed on the photosensitive drum 1d, and is transferred by the transfer member 53d, to obtain the single-color image. For preventing consumption of the photosensitive drums 1a, 1b, and 1c (used to form images of the other three colors), the photosensitive drums 1a, 1b, and 1c are stopped.
However, in the image forming apparatus, as also discussed in Japanese Patent Laid-Open No. 2002-173245, when the roller that regulates the position of the intermediate transfer belt is moved for separating the intermediate transfer belt from the photosensitive drums, a winding angle of the intermediate transfer belt 51 with respect to the steering roller 55 changes. This also changes the relationship between the inclination angle of the steering roller 55 and the magnitude of a force applied to the intermediate transfer belt 51 by the steering roller 55.
That is, as shown in FIG. 16, the winding angle with respect to the steering roller 55 is smaller when the intermediate transfer belt 51 (indicated by the broken line) is in contact with the photosensitive drums 1a to 1c than when the intermediate transfer belt 51 (indicated by the solid line) is separated from the photosensitive drums 1a to 1c. 
When the winding angle is reduced, the area of a portion of the intermediate transfer belt 51 that is wound upon the steering roller 55 is reduced, so that the force that the intermediate transfer belt 51 receives from the steering roller 55 is reduced. As a result, the pull of the belt is not quickly corrected, thereby making it difficult to overcome image distortion or color misregistration.
To overcome this problem, the inclination angle with respect to the steering roller 55 may be set large so that a sufficient amount of force is applied to the intermediate transfer belt 51 to correct the pull even if the winding angle of the intermediate transfer belt (indicated by the broken line) with respect to the steering roller 55 becomes small as a result of the intermediate transfer belt coming into contact with the photosensitive members.
However, in the case in which the correction of the pull is performed at the same inclination angle when the winding angle of the intermediate transfer belt 51 (indicated by the solid line) with respect to the steering roller 55 becomes large as a result of the intermediate transfer belt 51 being separated from the photosensitive drums 1, the force received by the intermediate transfer belt 51 from the steering roller 55 becomes too large. As a result, the life of the intermediate belt may be reduced due to, for example, streaks, folds, or breakage in a surface of the belt member resulting from material deterioration of the belt member.